Any outdoor carpet would work fine. That will give you something with a heavier/tighter weave that won't get caught on the edges/corners, sights of your guns. You can probably use what ever you like though. Shag carpet would be pimp, add a disco ball and some cheesy disco music and your guns will be dancing all night long! I'd pay money to see that.

Home Depot & Lowes usually sell outdoor carpet in 6' widths. They'll cut it to length for you, so you'll get 6 x whatever, then use a razor knife to trim to fit. It can be very inexpensive and they should have a few choices of styles and colors.
Also you could try a stereo/ car radio parts outlet as they should sell the heavy felt-like product that speaker box makers use. That's a good choice, too.